Re: [PATCH] sha1-file: remove OBJECT_INFO_SKIP_CACHED

From: Junio C Hamano <gitster@pobox.com>
Date: Dec 30, 2019, 21:43 UTC
Message-ID: <xmqqsgl1fq03.fsf@gitster-ct.c.googlers.com>
In-Reply-To: <20191230211027.37002-1-jonathantanmy@google.com>

Jonathan Tan <jonathantanmy@google.com> writes:

Show 7 quoted lines

> In a partial clone, if a user provides the hash of the empty tree ("git
> mktree </dev/null" - for SHA-1, this is 4b825d...) to a command which
> requires that that object be parsed, for example:
>
>   git diff-tree 4b825d <a non-empty tree>
>
> then Git will lazily fetch the empty tree.

That sounds like a bug. Shouldn't some objects like empty tree and empty blob whose names are hardcoded come internally without being fetched from anywhere?

Show 8 quoted lines

> There are 2 functions: repo_has_object_file() which does not consult
> find_cached_object() (which, among other things, knows about the empty
> tree); and repo_read_object_file() which does. This issue occurs
> because, as an optimization to avoid reading blobs into memory,
> parse_object() calls repo_has_object_file() before
> repo_read_object_file(). In the case of a regular repository (that is,
> not a partial clone), repo_has_object_file() will return false for the
> empty tree (thus bypassing the optimization) and...

OK, that bypassing already sounds wrong.

> ... it will no longer be the case that
> repo_has_object_file() doesn't know about empty trees, but
> repo_read_object_file() does.

Yup, that sounds like the right solution to the bug, with or without lazy cloning.